In the field of receiver and transmitter technology for mobile telecommunications to date, only receivers incorporating fixed analog intermediate frequency (IF) stages have been found suitable for use in base stations. This is because of the large range of power levels which must be handled, and also a requirement for a low level of blocking. The range of power levels is often called the dynamic range and is usually quantified as the ratio between strongest and weakest usable power levels. Blocking is, of course, the problem of a weaker signal being drowned out by a stronger signal.
Direct conversion receivers with zero or near-zero IF stages are becoming available. However they are only suitable for use in mobile user terminals but not base stations because power range and blocking requirements of mobile user terminals are much less demanding than those of base stations. Direct down-conversion receivers are known to suffer from the problem of direct current (DC) offsets in their IQ-mixers, which limits the useful dynamic range. Incidentally, IQ-mixers are also known in the art as I/Q modulators and quadrature mixers, where I refers to the in-phase component of a signal and Q refers to the quadrature phase component.
A so-called Othello chipset for mobile user terminals is known, which incorporates a zero IF direct conversion receiver for triple-band operation (GSM 900, 1800, 1900 MHz bands). A chipset for mobile user terminals is also known which operates with a near zero IF, namely 100 kHz. Such chipsets use Variable Gain Amplifiers (VGA) as part of an automatic gain control (AGC) loop to cope with the dynamic range requirements of the mobile user terminal. To compensate for the direct current (DC) offset, averaging over a long time period is used to estimate the DC-offset correction to be applied.
The DC-offset in a direct down-conversion receiver limits performance. Accurate detection is not possible when the DC-offset is stronger than the wanted signal as occurs with weak input signals. In consequence, the dynamic range of the analog to digital converter (ADC) used to sample the in-phase (I) and quadrature phase (Q) signal components is often insufficient; the dynamic range then being the ratio of the strongest received signal to the DC-offset rather than to the weakest received signal.
As regards transmitters, direct up-conversion can be used, although a high level of carrier signal suppression is often required. Direct up-conversion modulators each consisting of an IQ mixer are available in the marketplace. However, the powers used in base stations are often near to the minimum powers which these modulators can handle, i.e. the “noisefloor”. Such modulators typically offer carrier signal suppression in the order of 35 dB without tuning. However, to improve carrier suppression, a manual or automatic tuning process is used involving e.g. adjustment of variable resistors and/or capacitors so as to compensate for slight differences in gains and delays between the I and Q branches of the IQ mixer.
The limited carrier suppression capability of known direct up-conversion mixers limits their performance. Amplitude modulation applied to the I and Q input signals is limited by the carrier signal suppression because the minimum amplitude of each of the I and Q signals equals the level of the carrier after suppression. Also the suppressed carrier generates some distortion, called error vector magnitude (EVM), of the output signal. This distortion adds to the wanted signal as a rotating phasor or a displacement in the IQ-plane.